With ever-increasing demand for wireless communication and broadband services, there is an ongoing evolution of Third Generation (3G) and Fourth Generation (4G) cellular networks like High Speed Packet Access (HSPA), Evolution-Data Optimized (EV-DO), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX), International Mobile Telecommunications-Advanced (IMT-Advanced) (e.g., LTE Advanced), etc., to support ever-increasing performance with regard to capacity, peak bit rates and coverage. In case of a mobile communication environment, such as Third Generation Partnership Project's (3GPP) LTE network, the Evolved Universal Terrestrial Radio Access (EUTRA) or Evolved Universal Terrestrial Radio Access Network (E-UTRAN) air interface for LTE may support wireless broadband data service at a rate of up to 300 Mbps in the downlink (DL) and 75 Mbps in the uplink (UL).
One of the design targets of an LTE system is cost optimization, wherein complexity is reduced by focusing on only Internet Protocol (IP) capability—i.e., using data packets for all communication (whether voice call, text message, multimedia messages, etc.). In this packet-switched approach of LTE, a Circuit-Switched (CS) domain may not be supported when a Mobile Station (MS) or User Equipment (UE) is connected for LTE radio access. In that event, an IP Multimedia Subsystem (IMS) core may be needed (for packet data transmission over CS domain, and vice versa) even if the LTE UE is making a simple voice call. However, in early deployments of LTE, full LTE radio coverage may not be available across an operator's entire network or the operator may not have yet deployed an IMS core to support voice services over LTE. Thus, an LTE deployment may initially lack support for traditional mobile phone services like voice call, text message, multimedia messages vis-à-vis various other systems implemented on CS domain. Thus, to enable LTE to handle traditional phone services so as to be able to eventually replace current Second Generation (2G) and 3G networks, Circuit Switched Fallback (CSFB) was proposed to hand over voice calls from LTE to circuit-switched 2G/3G domains. The CSFB enables the delivery of CS-domain services by reuse of the CS infrastructure when a Mobile Station (MS) or User Equipment (UE) is normally served by another access technology, such as LTE.
It is known that a CS network is a type of network in which a physical path is obtained for and dedicated to a single connection between two end points in the network for the duration of the connection. A virtual CS connection (which may be used in a packet-switched network like LTE) is a dedicated logical connection that allows sharing of the physical path among multiple virtual circuit connections for packetized data transfer.
The CSFB approach allows network operators to guarantee voice service when a subscriber is connected to an LTE network. CSFB essentially kicks the phone off of LTE when traditional phone services are being used, and then flips the phone back to LTE when the voice call is over. Thus, voice calls by all-data LTE networks may be supported using CSFB to CS domains supported by other 2G/3G systems such as Wideband Code Division Multiple Access (WCDMA) systems, CDMA2000 systems, Global System for Mobile Communications (GSM) networks, etc., which support both voice and data at once anyway.
FIG. 1 illustrates an existing architecture 10 to enable CSFB from LTE-based E-UTRAN access to CDMA Single Carrier (1x) Radio Transmission Technology (RTT) CS domain access. The architecture 10 in FIG. 1 is discussed in more detail in 3GPP Technical Specification (TS) 23.272 v10.5.0 (titled “Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2”) and, hence, only a brief overview of the architecture that is relevant to the present disclosure is provided herein. The architecture 10 in FIG. 1 supports voice and other CS-domain services by reuse of the 1xCS infrastructure 12 (where various network elements may have circuit-switched connections therebetween) when a UE 14 is served by E-UTRAN 16 (in an LTE cell (not shown) in an EPC network domain 18). It is assumed here that the user terminal (MS or UE) 14 may be a dual-mode terminal capable of supporting E-UTRAN access as well as CSFB to 1xCS domain 12. Many details such as base stations or Evolved Node B's (eNB or eNodeB), network cells, etc., are not shown in FIG. 1 for the sake of simplicity and because of their lack of relevance to the present disclosure. In FIG. 1, the E-UTRAN 16 in the EPC domain 18 is shown connected to a gateway 20 via an S1-U interface (also referred to as “reference point” in relevant literature) to support packet data transfer and mobility of the UE 14 within the EPC network domain 18. The gateway 20 may be a Serving Gateway (S-GW) and/or Packet Data Network (PDN) Gateway (P-GW), which may be connected to an external IP network (e.g., the Internet) (not shown) via an SGi interface to provide the UE 14 in the LTE cell with seamless (and wireless) access to many different resources or systems beyond those operating within the operator's core network 18. A Mobility Management Entity (MME) 22 in the EPC network 18 may be configured to interface with an Interworking System (IWS) 24 in the 1xCS domain 12 via an S102 link or tunnel 25 to support CSFB to 1xRTT as discussed in more detail below. In other words, the CS fallback to 1xRTT is realized by using the S102 reference point 25 between the EPC network-based MME 22 and CS domain-based 1xCS IWS 24. The LTE radio access network (i.e., E-UTRAN 16 in FIG. 1) may connect to the MME 22 via an S1-MME reference point, whereas the gateway 20 may connect to the MME 22 via an S11 reference point as shown in FIG. 1.
The IWS 24 may be associated with a switch or 1xRTT Mobile Switching Center 27 via an A1 interface. It is observed here that an IWS may be logically co-located at a 1x Base Station Controller (BSC), co-located at the MSC 27, or may be a stand-alone entity (as shown, for example, in FIGS. 1-2). The reference numeral “29” in FIG. 1 may represent not only the BSC, but the Base Station (BS) as well. Hence, block 29 in FIG. 1 is generally labeled as “1xRTT CS Access.” If co-located, the IWS 24 may be with the BSC portion of the block 29 as stated earlier. In one embodiment, an IWS may be implemented as a function that supports CSFB solution for E-UTRAN to 1xCS fallback. The IWS may select a 1x BS that will serve the MS/UE for circuit-switched services. An IWS may provide functions such as, for example, translation between messages sent to or received from its associated MSC over the A1 interface and 1x air interface signaling messages received from or sent to tunneled air interfaces (e.g., E-UTRAN). As mentioned before, a 1x IWS may be linked to an EPC network-based entity (i.e., an MME) via an S102 reference point.
The MSC 27 may offer switching functionality to connect UE-originated calls to other public networks (e.g., Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), etc.) (not shown), other MSCs in the same network, or MSCs in different networks. The MSC 27 thus provides a CS interface for user traffic between the wireless network and other public switched networks or other MSCs. Although the discussion herein is primarily in the context of a circuit-switched MSC, it is understood that the MSC functionality may be divided so that the call control or mobility management and routing function may be implemented using a Mobile Switching Center emulation (MSCe), which may provide packet-based processing and control via IP based protocols. Thus, the MSC 27 may include such MSCe functionality as well.
In CSFB operation, initial mobile origination or paging (i.e., mobile termination) signaling may be sent between the user's terminal (i.e., the UE 14) and the legacy CS network (here, the 1xCS network 12) via a tunnel established from the LTE cell (or more correctly, from the EPC network 18) to an IWS in the legacy network (here, the IWS 24 in FIG. 1). In the configuration of FIG. 1, the S102 link 25 supports such tunnel, which is used to establish the initial call parameters. The UE 14 is then instructed (e.g., via the MME 22 and EUTRAN 16 in the EPC network 18) to fall back to the circuit-switched legacy network (as symbolically illustrated by arrow 30 in FIG. 1) to complete the connection of the bearer resources through the 1xRTT Access 29 and carry on the voice conversation. (In FIG. 1, dotted lines 31a and 31b symbolically illustrate wireless connections that exist between the UE 14 and the respective wireless base stations during CSFB.) At the end of the voice call, the UE 14 drops the CS connection and reconnects to the LTE network. In FIG. 1, the dotted arrow 32 illustrates the path of 1xRTT messages that are tunneled (via the S102 link 25) from the UE 14 (operating in the EPC network 18) to the MSC 27 (in the 1xCS network 12) during the time the UE 14 remains connected in the EPC network 18.